Electronically controlled solenoid operated valves provide improved shifting capabilities of a transmission compared to hydro-mechanical shift control and, in particular, permits staged or progressive release and application of clutches, such as band clutches and/or plate clutches, for effecting smoother speed changes in the transmission. In currently-known production transmissions, these valve assemblies are mounted internally on the transmission valve body and supplied with pressurized fluid from a pump disposed in the transmission. Shifting is performed using open-loop control of the valves, which requires time-consuming and costly calibration of each valve in the transmission system. However, even if the calibration is initially accurate, over time it may become less so due to the wear of parts in the transmission, degradation of transmission fluid, inherent nonlinearities in the system's behavior, etc., which in turn may negatively impact the shifting performance of the transmission. Moreover, present systems are inherently prone to error while in operation due to electrical load variations, pressure pulsations, temperature fluctuations, and other system non-linearities.
Closed-loop control of solenoid operated valves has been proposed as a way to obviate the need for precise calibration of the solenoid pilot and regulating valves used for clutch engagement and line pressure regulation in an automatic speed change transmission. Closed-loop control may include providing a feedback signal indicative of the torque transmitted by a particular shifting clutch band or plate to the solenoid valve. The pressure applied to the clutch actuator by the transmission hydraulic fluid is an indication of the clutch transmitted torque. Closed-loop control of a transmission system may be done by employing pressure transducers to sense the hydraulic pressure to each clutch actuator. An electrical signal corresponding to the detected pressure is transmitted to a transmission control unit (TCU). The transmission control unit then determines the difference between the actual sensed pressure and a target pressure corresponding to the desired output pressure to the actuators. The transmission control unit controls the current level or the duty-cycle sent to either a linear or pulse-width-modulated (PWM) solenoid-operated valve to control the clutch regulator valve or the clutch actuator directly until the actual pressure reaches the target pressure.
Accurate control of the closed-loop transmission system can be affected how precisely the hydraulic pressure applied to the clutch actuator can be measured. Piezoelectric pressure transducers are often a viable choice in many pressure sensing applications because of their low cost. Their use, however, does impose potential design challenges due to their non-linear response, low output signal strength, and temperature sensitivity. For example, piezoelectric pressure transducers typically require signal amplification and correction for non-linearities. Further, piezoelectric pressure transducers may be sensitive to internal stresses that can be erroneously detected by the pressure transducer as a pressure. Undesirable and often unavoidable, internal stresses may occur when the pressure transducer is attached to another component. The extreme temperature conditions that the pressure transducers are subjected to may also produce thermal stresses within the pressure transducer that may produce a false pressure reading. These and other operating conditions may detrimentally impact the accuracy of the pressure transducers.